Resolve regression issues related to foreign keys

[aeneasr]

Client credentials flow

sh-4.3$ docker exec -it hydra_hydra_1 \
>     hydra clients create \
>     --endpoint http://localhost:4445 \
>     --id my-client \
>     --secret secret \
>     -g client_credentials
You should not provide secrets using command line flags, the secret might leak to bash history and similar systems
OAuth 2.0 Client ID: my-client
sh-4.3$ docker exec -it hydra_hydra_1 \
>     hydra token client \
>     --endpoint http://localhost:4444 \
>     --client-id my-client \
>     --client-secret secret
Could not retrieve access token because: oauth2: cannot fetch token: 500 Internal Server Error
Response: {"error":"error","error_description":"The error is unrecognizable.","status_code":500,"error_debug":"pq: insert or update on table \"hydra_oauth2_access\" violates foreign key constraint \"hydra_oauth2_access_request_id_fk\""}

Cause:

• Client credentials flow does not use request_id's - we should be able to set it to NULL.